Carbon stocks and soil respiration rates during deforestation, grassland use and subsequent Norway spruce afforestation in the Southern Alps, Italy

Changes in carbon stocks during deforestation, reforestation and afforestat ion play an important role in the global carbon cycle. Cultivation of fores t lands leads to substantial losses in both biomass and soil carbon, wherea s forest regrowth is considered to be a significant carbon sink. We examine d below- and aboveground carbon stocks along a chronosequence of Norway spr uce (Picea abies (L.) Karst.) stands (0-62 years old) regenerating on aband oned meadows in the Southern Alps. A 130-year-old mixed coniferous Norway s pruce-white fir (Abies alba Mill.) forest, managed by selection cutting, wa s used as an undisturbed control. Deforestation about 260 years ago led to carbon losses of 53 Mg C ha(-1) from the organic layer and 12 Mg C ha(-1) f rom the upper mineral horizons (A(h) E). During the next 200 years of grass land use, the new Ah horizon sequestered 29 Mg C ha(-1). After the abandonm ent of these meadows, carbon stocks in tree stems increased exponentially d uring natural forest succession, levelling off at about 190 Mg C ha(-1) in the 62-year-old Norway spruce and the 130-year-old Norway spruce-white fir stands. In contrast, carbon stocks in the organic soil layer increased line arly with stand age. During the first 62 years, carbon accumulated at a rat e of 0.36 Mg C ha(-1) L year(-1) in the organic soil layer. No clear trend with stand age was observed for the carbon stocks in the Ah horizon. Soil r espiration rates were similar for all forest stands independently of organi c layer thickness or carbon stocks, but the highest rates were observed in the cultivated meadow. Thus, increasing litter inputs by forest vegetation compared with the meadow, and constantly low decomposition rates of conifer ous litter were probably responsible for continuous soil carbon sequestrati on during forest succession. Carbon accumulation in woody biomass seemed to slow down after 60 to 80 years, but continued in the organic soil layer. W e conclude that, under present climatic conditions, forest soils act as mor e persistent carbon sinks than vegetation that will be harvested, releasing the carbon sequestered during tree growth.